Older adults with mild cognitive impairment showed reduced heart rate variability overnight, most sharply around midnight, compared with cognitively normal peers wearing the same seven-day ambulatory ECG monitors, according to a study published January 5, 2026 in Frontiers in Aging Neuroscience. The difference at that hour reached a Cohen's d of 0.75, a moderate to large effect for this kind of physiological measure, with heart rate variability also lower in the early morning before 7 a.m. and evening after 7 p.m.
The research, drawn from the Einstein Aging Study in the Bronx, addresses a gap that has dogged this field: most prior work measured heart rate variability from a single brief clinic recording, producing one averaged number that could not show when in the day, or night, any difference actually appeared. Researchers led by Jiyue Qin and Carol Derby applied a statistical method called functional data analysis, along with functional additive mixed models, to trace heart rate variability continuously across a full 24 hour cycle rather than reducing it to an average, after adjusting for age, sex, ethnicity, education, depression, diabetes and hypertension.
The cohort was small and the design cross-sectional: 81 dementia-free, community-dwelling adults, mean age 78, of whom only 20 met criteria for mild cognitive impairment and 61 were cognitively normal. A single snapshot in time, comparing two groups at one point, cannot establish that the nighttime pattern preceded the cognitive difference or that it will predict future decline; the authors themselves describe the finding as underscoring heart rate variability's dynamic, time-of-day-dependent nature rather than as a validated marker of anything.